[webrtc-pc] new commits pushed by aboba

The following commits were just pushed by aboba to https://github.com/w3c/webrtc-pc:

* addTransceiver: check [[isClosed]]

adds a step to check [[isClosed]] similar to what is done for addTrack

fixes #1891
  by Philipp Hancke
https://github.com/w3c/webrtc-pc/commit/58bedda30507c730ffdc6a219c1729d433fd1037

* Merge pull request #1893 from fippo/addtransceiver-closed

addTransceiver: check [[isClosed]]
  by Bernard Aboba
https://github.com/w3c/webrtc-pc/commit/485bc92ffee3077bb07fadbad4ab3e04e8a88c36